Publications

2012

• New Insight into Short-wavelength Solar Wind Fluctuations from Vlasov Theory
  
  • Sahraoui Fouad
  • Belmont Gérard
  • Goldstein M. L.
  The Astrophysical Journal, American Astronomical Society, 2012, 748, pp.100. The nature of solar wind (SW) turbulence below the proton gyroscale is a topic that is being investigated extensively nowadays, both theoretically and observationally. Although recent observations gave evidence of the dominance of kinetic Alfvén waves (KAWs) at sub-ion scales with omega < omega<SUB>ci</SUB>, other studies suggest that the KAW mode cannot carry the turbulence cascade down to electron scales and that the whistler mode (i.e., omega > omega<SUB>ci</SUB>) is more relevant. Here, we study key properties of the short-wavelength plasma modes under limited, but realistic, SW conditions, typically beta<SUB> i </SUB> >~ beta<SUB> e </SUB> ~ 1 and for high oblique angles of propagation 80° <= Theta<SUB> kB </SUB> < 90° as observed from the Cluster spacecraft data. The linear properties of the plasma modes under these conditions are poorly known, which contrasts with the well-documented cold plasma limit and/or moderate oblique angles of propagation (Theta<SUB> kB </SUB> < 80°). Based on linear solutions of the Vlasov kinetic theory, we discuss the relevance of each plasma mode (fast, Bernstein, KAW, whistler) in carrying the energy cascade down to electron scales. We show, in particular, that the shear Alfvén mode (known in the magnetohydrodynamic limit) extends at scales krho<SUB> i </SUB> >~ 1 to frequencies either larger or smaller than omega<SUB>ci</SUB>, depending on the anisotropy k <SUB>par</SUB>/k <SUB></SUB>. This extension into small scales is more readily called whistler (omega > omega<SUB>ci</SUB>) or KAW (omega < omega<SUB>ci</SUB>), although the mode is essentially the same. This contrasts with the well-accepted idea that the whistler branch always develops as a continuation at high frequencies of the fast magnetosonic mode. We show, furthermore, that the whistler branch is more damped than the KAW one, which makes the latter the more relevant candidate to carry the energy cascade down to electron scales. We discuss how these new findings may facilitate resolution of the controversy concerning the nature of the small-scale turbulence, and we discuss the implications for present and future spacecraft wave measurements in the SW. (10.1088/0004-637X/748/2/100)
  DOI : 10.1088/0004-637X/748/2/100
• Control of the ion flux and ion energy in CCP discharges using non-sinusoidal voltage waveforms
  
  • Lafleur Trevor
  • Booth Jean-Paul
  Journal of Physics D: Applied Physics, IOP Publishing, 2012, 45, pp.395203. Using particle-in-cell simulations we perform a characterization of the ion flux and ion energy in a capacitively coupled rf plasma reactor excited with non-sinusoidal voltage waveforms. The waveforms used are positive Gaussian type pulses (with a repetition frequency of 13.56 MHz), and as the pulse width is decreased, three main effects are identified that are not present in typical symmetric sinusoidal discharges: (1) the ion flux (and plasma density) rapidly increases, (2) as the pressure increases a significant asymmetry in the ion fluxes to the powered and grounded electrodes develops and (3) the average ion energy on the grounded electrode cannot be made arbitrarily small, but in fact remains essentially constant (together with the bias voltage) for the pressures investigated (20500 mTorr). Effects (1) and (3) potentially offer a new form of control in these types of rf discharges, where the ion flux can be increased while keeping the average ion energy on the grounded electrode constant. This is in contrast with the opposite control mechanism recently identified in Donkó et al (2009 J. Phys. D: Appl. Phys. 42 025205), where by changing the phase angle between applied voltage harmonics the ion flux can be kept constant while the ion energy (and bias voltage) varies. (10.1088/0022-3727/45/39/395203)
  DOI : 10.1088/0022-3727/45/39/395203
• Hydrogenated microcrystalline silicon thin films deposited by RF-PECVD under low ion bombardment energy using voltage waveform tailoring
  
  • Johnson E.V.
  • Pouliquen S.
  • Delattre Pierre-Alexandre
  • Booth Jean-Paul
  Journal of Non-Crystalline Solids, Elsevier, 2012, 358 (17), pp.1974-1977. We present experimental results for hydrogenated amorphous and microcrystalline silicon (a-Si:H and &#956;c-Si:H) thin films deposited by PECVD while using a voltage waveform tailoring (VWT) technique to create an electrical asymmetry in the reactor. VWT dramatically modifies the mean ion bombardment energy (IBE) during growth, and we show that for a constant peak-to-peak excitation voltage (VPP), waveforms resembling peaks or valleys result in very different material properties. Using Raman scattering spectroscopy, we show that the crystallinity of the material depends strongly on the IBE, as controlled by VWT. A detailed examination of the Raman scattering spectra reveals that the narrow peak at 520 cm&#8722; 1 is disproportionately enhanced by lowering the IBE through the VWT technique. We examine this effect for a range of process parameters, varying the pressure, hydrogensilane dilution ratio, and total flow of H2. In addition, the Sisingle bondHX bonding in silicon thin films deposited using VWT is characterised for the first time, showing that the hydrogen bonding character is changed by the IBE. These results demonstrate the potential for VWT in controlling the IBE during thin film growth, thus ensuring that application-appropriate film densities and crystallinities are achieved, independent of the injected RF power. (10.1016/j.jnoncrysol.2012.01.014)
  DOI : 10.1016/j.jnoncrysol.2012.01.014
• Model of a low-pressure radio-frequency inductive discharge in Ar/O<SUB>2</SUB> used for plasma spray deposition
  
  • Lazzaroni Claudia
  • Baba K.
  • Nikravech M.
  • Chabert Pascal
  Journal of Physics D: Applied Physics, IOP Publishing, 2012, 45, pp.485207. A global (volume-averaged) model of a low-pressure radio-frequency (RF) inductive discharge used for nanostructured zinc oxide thin film deposition, the so-called spray-plasma device, is proposed. The plasma reactor is fed with an admixture of argon and oxygen and the pressure is typically several tens of mTorr. In the first step of the modelling, the injector and the substrate holder are not taken into account, and therefore zinc-containing species are not considered. The global model is based on the numerical integration of the particle balance equations and the electronic power balance equation. The model is first run until the steady state is reached to determine the equilibrium discharge parameters that are the species densities and the electron temperature. A parametric study is carried out varying the gas pressure, the RF power and the O2 fraction in the reactor. A parameter of great importance for the deposition process is the flux of the reactive species on the substrate holder and the model allows a fast exploration of this parameter. For continuous plasmas, the ratio of the reactive species flux to the total positive ion flux can be controlled varying the three basic parameters cited before (pressure, power and dilution). In the last part of the paper, we also investigate pulsed plasmas and the effect of the duty cycle variations on the neutral/ion flux ratio. (10.1088/0022-3727/45/48/485207)
  DOI : 10.1088/0022-3727/45/48/485207
• ON THE NATURE OF THE SOLAR WIND FROM CORONAL PSEUDOSTREAMERS
  
  • Wang Y-M
  • Grappin Roland
  • Robbrecht E.
  • Sheeley N R
  The Astrophysical Journal, American Astronomical Society, 2012, 749 (2), pp.182. Coronal pseudostreamers, which separate like-polarity coronal holes, do not have current sheet extensions, unlike the familiar helmet streamers that separate opposite-polarity holes. Both types of streamers taper into narrow plasma sheets that are maintained by continual interchange reconnection with the adjacent open magnetic field lines. White-light observations show that pseudostreamers do not emit plasma blobs; this important difference from helmet streamers is due to the convergence of like-polarity field lines above the X-point, which prevents the underlying loops from expanding outward and pinching off. The main component of the pseudostreamer wind has the form of steady outflow along the open field lines rooted just inside the boundaries of the adjacent coronal holes. These flux tubes are characterized by very rapid expansion below the X-point, followed by reconvergence at greater heights. Analysis of an idealized pseudostreamer configuration shows that, as the separation between the underlying holes increases, the X-point rises and the expansion factor f ss at the source surface increases. In situ observations of pseudostreamer crossings indicate wind speeds v ranging from ~350 to ~550 km s1, with O7 /O6 ratios that are enhanced compared with those in high-speed streams but substantially lower than in the slow solar wind. Hydrodynamic energy-balance models show that the empirical v-f ss relation overestimates the wind speeds from nonmonotonically expanding flux tubes, particularly when the X-point is located at low heights and f ss is small. We conclude that pseudostreamers produce a "hybrid" type of outflow that is intermediate between classical slow and fast solar wind. (10.1088/0004-637X/749/2/182)
  DOI : 10.1088/0004-637X/749/2/182
• A hemispherical retarding field energy analyzer to characterize spatially and angularly extended electron beams
  
  • Cipriani Fabrice
  • Leblanc Frédéric
  • Illiano Jean-Marie
  • Berthelier Jean-Jacques
  European Physical Journal: Applied Physics, EDP Sciences, 2012, 60 (2), pp.21002 (7 p.). We have designed and built a hemispherical retarding field energy analyzer in order to facilitate characterization of large area electron emitters (typically field emitter arrays with active areas up to 1 cm2) with large angular aperture. A complete numerical model of the analyzer has been built, including perturbations due to secondary particles, in order to determine the analyzer performances. The analyzer energy resolution is better than 100 meV for an energy range up to 120 eV. The analyzer has a global field of view of 112° and allows measurements of the energy distribution of the beam as a function of the emission angle, as well as measurements of the beam intensity profile along any section of the beam. We have successfully used the analyzer to characterize the electron beam emitted by 1 cm2 Mo microtips-based field emitter arrays. (10.1051/epjap/2012120011)
  DOI : 10.1051/epjap/2012120011
• Kinetic equilibrium for an asymmetric tangential layer, Physics of Plasmas
  
  • Belmont Gérard
  • Aunai Nicolas
  • Smets Roch
  Physics of Plasmas, American Institute of Physics, 2012, 19, pp.022108. Finding kinetic (Vlasov) equilibria for tangential current layers is a long standing problem, especially in the context of reconnection studies, when the magnetic field reverses. Its solution is of pivotal interest for both theoretical and technical reasons when such layers must be used for initializing kinetic simulations. The famous Harris equilibrium is known to be limited to symmetric layers surrounded by vacuum, with constant ion and electron flow velocities, and with current variation purely dependent on density variation. It is clearly not suited for the magnetopause-like layers, which separate two plasmas of different densities and temperatures, and for which the localization of the current density j=n&#948;v is due to the localization of the electron-to-ion velocity difference &#948;v and not of the density n. We present here a practical method for constructing a Vlasov stationary solution in the asymmetric case, extending the standard theoretical methods based on the particle motion invariants. We show that, in the case investigated of a coplanar reversal of the magnetic field without electrostatic field, the distribution function must necessarily be a multi-valued function of the invariants to get asymmetric profiles for the plasma parameters together with a symmetric current profile. We show also how the concept of accessibility makes these multi-valued functions possible, due to the particle excursion inside the layer being limited by the Larmor radius. In the presented method, the current profile across the layer is chosen as an input, while the ion density and temperature profiles in between the two asymptotic imposed values are a result of the calculation. It is shown that, assuming the distribution is continuous along the layer normal, these profiles have always a more complex profile than the profile of the current density and extends on a larger thickness. The different components of the pressure tensor are also outputs of the calculation and some conclusions concerning the symmetries of this tensor are pointed out. (10.1063/1.3685707)
  DOI : 10.1063/1.3685707
• Analysis of radiation from silver HED plasma sources with the potential for lasing
  
  • Weller Michael E.
  • Safronova Alla S.
  • Kantsyrev Viktor L.
  • Esaulov A. A.
  • Stafford A.
  • Shrestha Ishor
  • Osborne Glenn C.
  • Shlyaptseva V. V.
  • Keim S. F.
  • Zunino H. A.
  • Chuvatin Alexandre S.
  • Apruzese J. P.
  • Golovkin I. E.
  • Macfarlane J. J.
  , 2012, pp.2P78. Silver (Ag) high energy density plasmas were produced using uniform and combined single planar wire array (SPWA) z-pinches. Ag SPWAs were recently introduced as an efficient x-ray radiator and have shown to create L-shell plasmas that have the highest electron temperature (>;1.8 keV) observed on Zebra so far and upwards of 30 kJ of energy output, which is of interest for future applications of inertial confinement fusion1. A set of diagnostics included fast, filtered x-ray diodes; a Ni bolometer; laser shadowgraphy and optical streak cameras; time-gated and time-integrated x-ray pinhole cameras; and time-integrated spatially resolved (TISR) and time-gated spatially-integrated (TGSI) x-ray spectrometers. In particular, a new time-gated hard x-ray spectrometer was fielded to attain first results to understand how Ag plasmas evolve in time. In addition, an important question about such Ag plasmas is whether lasing occurs in the Na-like and Ne-like soft x-ray range, and if so, at what gains? Our suite of theoretical diagnostics was expanded with HELIOS-CR code, that was utilized to study implosion characterisitcs and radiative characteristics of Ag wire arrays as well as to calculate possible lasing gains. Lastly, the results of new experiments on Zebra with the load current multiplier (LCM) at enhanced current of 1.5 - 1.7 MA were analyzed and compared to those of standard configurations. (10.1109/PLASMA.2012.6383634)
  DOI : 10.1109/PLASMA.2012.6383634
• Centrifugally stimulated exospheric ion escape at Mercury
  
  • Delcourt Dominique C.
  • Seki K.
  • Terada N.
  • Moore T. E.
  Geophysical Research Letters, American Geophysical Union, 2012, 39, pp.22105. We investigate the transport of ions in the low-altitude magnetosphere of Mercury. We show that, because of small spatial scales, the centrifugal effect due to curvature of the E × B drift paths can lead to significant particle energization in the parallel direction. We demonstrate that because of this effect, ions with initial speed smaller than the escape speed such as those produced via thermal desorption can overcome gravity and escape into the magnetosphere. The escape route of this low-energy exosphere originating material is largely controlled by the magnetospheric convection rate. This escape route spreads over a narrower range of altitudes when the convection rate increases. Bulk transport of low-energy planetary material thus occurs within a limited region of space once moderate magnetospheric convection is established. These results suggest that, via release of material otherwise gravitationally trapped, the E × B related centrifugal acceleration is an important mechanism for the net supply of plasma to the magnetosphere of Mercury. (10.1029/2012GL054085)
  DOI : 10.1029/2012GL054085
• Velocity diffusion in plasma waves excited by electron beams
  
  • Volokitin A.
  • Krafft C.
  Plasma Physics and Controlled Fusion, IOP Publishing, 2012, 54, pp.085002. New results provided by numerical simulations of the weak instability of a warm electron beam in a collisionless plasma are presented. The theoretical model considers the self-consistent resonant interactions of beam particles with wave packets of broad spectra; it is derived using some of the initial approximations of the standard derivation of the quasilinear diffusion equation in the weak turbulence approach, without, however, the assumption of randomly phased waves. A huge number of particle trajectories calculated over long times by a symplectic code are analyzed using various statistical algorithms. The dynamics of the beam relaxation and the saturation of the wave spectrum are studied and compared with the analytical solutions provided by the quasilinear theory of weak turbulence. The most interesting results concern the presence of strong and persistent irregularities in the wave energy spectrum at saturation, which are linked to large velocity variations observed in the particles' dynamics and to non-Gaussian local diffusion. Quantitative estimates of the diffusion coefficients are given and compared with predictions of the weak turbulence theory. (10.1088/0741-3335/54/8/085002)
  DOI : 10.1088/0741-3335/54/8/085002
• THEMIS observation of chorus elements without a gap at half the gyrofrequency
  
  • Kurita S.
  • Katoh Y.
  • Omura Y.
  • Angelopoulos V.
  • Cully C. M.
  • Le Contel Olivier
  • Misawa H.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2012, 117, pp.11223. Using waveform data obtained by one of the THEMIS satellites, we report properties of rising tone chorus elements without a gap at half the gyrofrequency in a region close to the magnetic equator. The wave normal angle of the chorus elements is typically field-aligned in the entire frequency range of both upper-band and lower-band chorus emissions. We find that the observed frequency sweep rates are consistent with the estimation based on the nonlinear wave growth theory of Omura et al. (2008). In addition, we compare the frequency profiles of the chorus wave amplitudes with those of the optimum and threshold wave amplitudes derived from the nonlinear wave growth theory for triggering rising tone chorus emissions. The results of the comparison show a reasonable agreement, indicating that rising tone chorus elements are continually generated through a triggering process which generates elements with the optimum amplitudes for nonlinear growth. (10.1029/2012JA018076)
  DOI : 10.1029/2012JA018076
• An electromagnetic theory of turbulence driven poloidal rotation
  
  • Mcdevitt C.J.
  • Gürcan Özgür D.
  Physics of Plasmas, American Institute of Physics, 2012, 19, pp.102311. An electromagnetic theory of turbulence driven poloidal rotation is developed with particular emphasis on understanding poloidal rotation in finite-&#946; plasmas. A relation linking the flux of polarization charge to the divergence of the total turbulent stress is derived for electromagnetic gyrokinetic modes. This relation is subsequently utilized to derive a constraint on the net electromagnetic turbulent stress exerted on the poloidal flow. Various limiting cases of this constraint are considered, where it is found that electromagnetic contributions to the turbulent stress may either enhance or reduce the net turbulent stress depending upon the branch of turbulence excited. (10.1063/1.4764078)
  DOI : 10.1063/1.4764078
• Quasilinear transport modelling at low magnetic shear
  
  • Citrin Jonathan
  • Bourdelle C.
  • Cottier P.
  • Escande D.F.
  • Gürcan Özgür D.
  • Hatch D.R.
  • Hogeweij G.M.D.
  • Jenko F.
  • Pueschel M.J.
  Physics of Plasmas, American Institute of Physics, 2012, 19, pp.062305. Accurate and computationally inexpensive transport models are vital for routine and robust predictions of tokamak turbulent transport. To this end, the QuaLiKiz [Bourdelle et al., Phys. Plasmas 14, 112501 (2007)] quasilinear gyrokinetic transport model has been recently developed. QuaLiKiz flux predictions have been validated by non-linear simulations over a wide range in parameter space. However, a discrepancy is found at low magnetic shear, where the quasilinear fluxes are significantly larger than the non-linear predictions. This discrepancy is found to stem from two distinct sources: the turbulence correlation length in the mixing length rule and an increase in the ratio between the quasilinear and non-linear transport weights, correlated with increased non-linear frequency broadening. Significantly closer agreement between the quasilinear and non-linear predictions is achieved through the development of an improved mixing length rule, whose assumptions are validated by non-linear simulations. (10.1063/1.4719697)
  DOI : 10.1063/1.4719697
• Symmetry breaking effects of density gradient on parallel momentum transport: A new rau s * effect
  
  • Singh Rameswar
  • Singh R.
  • Kaw P.
  • Gürcan Özgür D.
  • Diamond P.H.
  • Nordman H.
  Physics of Plasmas, American Institute of Physics, 2012, 19, pp.012301. Symmetry breaking effects of density gradient on parallel momentum transport is studied via quasilinear theory. It is shown that finite &#961;*s(&#8801;&#961;s/Ln), where &#961;s is ion sound radius and Ln is density scale length, leads to symmetry breaking of the ion temperature gradient (ITG) eigenfunction. This broken symmetry persists even in the absence of mean poloidal (from radial electric field shear) and toroidal flows. This effect, as explained in the text, originates from the divergence of polarization particle current in the ion continuity equation. The form of the eigenfunction allows the microturbulence to generate parallel residual stress via &#12296;k&#8214;&#12297; symmetry breaking. Comparison with the &#8594;E×&#8594;B shear driven parallel residual stress, parallel polarization stress and turbulence intensity gradient driven parallel residual stress are discussed. It is shown that this &#961;*s driven parallel residual stress may become comparable to &#8594;E×&#8594;B shear driven parallel residual stress in small Ln region. In the regular drift wave ordering, where &#961;*s&#8810;1, this effect is found to be of the same order as the parallel polarization stress. This &#961;*s driven parallel residual stress can also overtake the turbulence intensity gradient driven parallel residual stress in strong density gradient region whereas the later one is dominant in the strong profile curvature region. The parallel momentum diffusivity is found to remain undisturbed by this &#961;*s effect as long as the turbulence intensity inhomogenity is not important. (10.1063/1.3672518)
  DOI : 10.1063/1.3672518
• Electron energy distribution function and plasma parameters across magnetic filters
  
  • Aanesland Ane
  • Bredin Jérôme
  • Chabert Pascal
  • Godyak V.
  Applied Physics Letters, American Institute of Physics, 2012, 100, pp.044102. The electron energy distribution function (EEDF) is measured across a magnetic filter in inductively coupled plasmas. The measured EEDFs are found to be Maxwellian in the elastic energy range with the corresponding electron temperature monotonously decreasing along the positive gradient of the magnetic field. At the maximum of the magnetic field, the electron temperature reaches its minimum and remains nearly constant in the area of the negative gradient of the field, where the plasma density distribution exhibits a local minimum. (10.1063/1.3680088)
  DOI : 10.1063/1.3680088
• Dynamics of neutral gas depletion investigated by time- and space-resolved measurement of xenon atom ground state density
  
  • Liard Laurent
  • Aanesland Ane
  • Chabert Pascal
  Journal of Physics D: Applied Physics, IOP Publishing, 2012, 45, pp.235201. The dynamics of neutral gas depletion in high-density plasmas is investigated by time- and space-resolved measurements of the xenon ground state density. Two-photon absorbed laser induced fluorescence experiments were carried out in a helicon reactor operating at 10 mTorr in xenon gas. When the plasma is magnetized, a plasma column is formed from the bottom of the chamber up to the pumping region. In this situation it is found that two phenomena, with different time scales, are responsible for the neutral gas depletion. The magnetized plasma column is ignited in a short (millisecond) time scale leading to a neutral gas depletion at the discharge centre and to an increase of neutral gas density at the reactor walls. This is explained both by neutral gas heating and by the rise of the plasma pressure at the discharge centre. Then, on a much longer (second) time scale, the overall neutral gas density in the reactor decreases due to higher pumping efficiency when the magnetized plasma column is ignited. The pumping enhancement is not observed when the plasma is not magnetized, probably because in this case the dense plasma column vanishes and the plasma is more localized near the antenna. (10.1088/0022-3727/45/23/235201)
  DOI : 10.1088/0022-3727/45/23/235201
• Diagnostic emission and absorption spectroscopy
  
  • Roepcke J.
  • Rousseau Antoine
  • Lavrov B.P.
  • Davies P.B.
  , 2012.
• Dynamic Martian magnetosphere: Transient twist induced by a rotation of the IMF
  
  • Modolo Ronan
  • Chanteur Gérard
  • Dubinin Eduard
  Geophysical Research Letters, American Geophysical Union, 2012, 39 (1), pp.L01106. Simulation studies of the Martian environment are usually restricted to stationary situations under various steady conditions of the solar wind and solar radiation. Dynamic transients and their implications have so far attracted little attention although global simulation models can provide valuable insights to understand disagreements between simulations and in situ observations. We make use of a three dimensional multispecies hybrid simulation model to investigate the response of the Martian plasma environment to a sudden rotation of the IMF. The simulation model couples charged and neutral species via three ionisation mechanisms: the absorption of solar extreme ultraviolet radiation, the impact of solar wind electrons, and the charge exchange between ions and neutral atoms. When a rotational discontinuity conveyed by the solar wind reaches the Martian environment the bow shock adapts quickly to the new solar wind conditions in contrast to the induced magnetosphere, especially the magnetic lobes in the wake. Timescales necessary to recover a stationary state can be estimated from such simulations and have some implications for space observations especially in the use of magnetic field proxies and for organizing particle measurements made by a spacecraft like Mars Express without an onboard magnetometer. (10.1029/2011GL049895)
  DOI : 10.1029/2011GL049895
• On time resolved gas temperature measurements in a pulsed dc plasma using quantum cascade laser absorption spectroscopy
  
  • Hübner M.
  • Marinov Daniil
  • Guaitella Olivier
  • Rousseau Antoine
  • Roepcke J.
  Measurement Science and Technology, IOP Publishing, 2012, 23, pp.115602. With a time resolution of 33 µs, the gas temperature in a pulsed dc air plasma admixed with 0.8% NO has been measured by quantum cascade laser absorption spectroscopy (QCLAS). For this purpose, the temperature dependent intensity ratios of two absorption structures of NO at 1900 cm&#8722;1 (5.26 µm) have been used. The QCLAS system worked in the Intra Pulse Mode with a pulse repetition frequency of 30 kHz leading to a spectrum recorded each 33 µs. In a low pressure discharge, the influence of nonlinear absorption phenomena causing strong distorted absorption structures of NO has been taken into account by a calibration routine based on tabulated line strengths. Different mean plasma currents have been applied to the discharge leading to gas temperature values ranging from about 300 K up to about 500 K. (10.1088/0957-0233/23/11/115602)
  DOI : 10.1088/0957-0233/23/11/115602
• Plasma decay in the afterglow of a high-voltage nanosecond discharge in air
  
  • Aleksandrov N.L.
  • Anokhin E.M.
  • Kindysheva S.V.
  • Kirpichnikov A.A.
  • Kosarev I.N.
  • Nudnova M.M.
  • Starikovskaia Svetlana
  • Starikovskii A.Yu.
  Fizika Plazmy / Plasma Physics Reports, MAIK Nauka/Interperiodica, 2012, 38 (2), pp.179-186. The decay of air plasma produced by a high-voltage nanosecond discharge at room temperature and gas pressures in the range of 110 Torr was studied experimentally and theoretically. The time dependence of the electron density was measured with a microwave interferometer. The initial electron density was about 10¹2 cm3. The discharge homogeneity was monitored using optical methods. The dynamics of the charged particle densities in the discharge afterglow was simulated by numerically solving the balance equations for electron and ions and the equation for the electron temperature. It was shown that, under these experimental conditions, plasma electrons are mainly lost due to dissociative and three-body recombination with ions. Agreement between the measured and calculated electron densities was achieved only when the rate constant of the three-body electronion recombination was increased by one order of magnitude and the temperature dependence of this rate constant was modified. This indicates that the mechanism for three- body recombination of molecular ions differs from that of the well-studied mechanism of atomic ion recombination. (10.1134/S1063780X12010011)
  DOI : 10.1134/S1063780X12010011
• Electron and wave characteristics observed by the THEMIS satellites near the magnetic equator during a pulsating aurora
  
  • Nakajima A.
  • Shiokawa K.
  • Sakaguchi K.
  • Miyoshi Y.
  • Lee S.
  • Angelopoulos V.
  • Le Contel Olivier
  • Mcfadden J. P.
  • Bonnell J. W.
  • Fornacon K.-H.
  • Donovan E.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2012, 117, pp.3219. Based on conjugate ground and THEMIS satellite observations, we show electron spectra and wave characteristics near the magnetic equatorial plane during a pulsating aurora event on the high latitude side of the auroral oval. The pulsating aurora was observed by a 30-Hz sampled all-sky imager (ASI) at Gillam (56.4°N, 265.4°E), Canada, at 0840-0910 UT on 8 January 2008. The auroral intensity pulsation at the possible THEMIS D (THD) footprints had frequency peaks at 0.1-0.2 Hz. The footprint of THD was in the poleward part of the proton aurora observed by a meridian-scanning photometer. After auroral pulsation began at 0842 UT, both THD and THEMIS E which was near THD in the mid-tail at 11.6-11.8 R<SUB>E</SUB>, observed enhanced field-aligned electron fluxes at energies of 1-10 keV. However, the amplitudes of whistler mode waves and electrostatic cyclotron harmonics (ECH) waves observed by THD with the highest sampling rate of 8 kHz were not significant, showing a marked contrast to the recent report of clear correlation between whistler mode waves and auroral pulsations observed at 5-9 R<SUB>E</SUB>. We suggest that the observed field-aligned electrons, which are probably caused by Fermi-type acceleration associated with earthward plasma flow in the mid-tail plasma sheet, are modulated by some wave processes to cause pulsating auroras. (10.1029/2011JA017066)
  DOI : 10.1029/2011JA017066
• Tailored voltage waveform deposition of Microcrystalline Silicon thin films from hydrogen-diluted silane and silicon tetrafluoride: optoelectronic properties of films
  
  • Johnson E.V.
  • Pouliquen S.
  • Delattre Pierre-Alexandre
  • Booth Jean-Paul
  Japanese Journal of Applied Physics, Japan Society of Applied Physics, 2012, 51, pp.08HF01. The use of tailored voltage waveforms (TVW's) to excite a plasma for the deposition of thin films of hydrogenated microcrystalline silicon (µc-Si:H) has been shown to be an effective technique to decouple mean ion bombardment energy (IBE) from injected power. In this work, we examine the changes in material properties controlled by this technique through Raman scattering and spectroscopic ellipsometry for films deposited from H2-diluted SiH4, and we examine the electrical properties of such films using temperature dependent conductivity. As the laboratory-scale deposition system used had neither a load lock nor an oxygen filter in the H2 line, accidental O-doping was observed for the µc-Si:H films. We investigated suppression of this doping by adding varying amounts of SiF4, and using an SiF4/Ar pre-etch step to clean the reactor. This technique is shown to be effective in decreasing the accidental doping of the films, and intrinsic µc-Si:H films are produced with an activation energy of up to 0.55 eV. As well, an important difference in the amorphous-to-microcrystalline transition is observed once SiF4 is included in the gas mixture. (10.1143/JJAP.51.08HF01)
  DOI : 10.1143/JJAP.51.08HF01
• Comparison of a hybrid model to a global model of atmospheric pressure radio-frequency capacitive discharges
  
  • Lazzaroni Claudia
  • Lieberman M.A.
  • Lichtenberg A.J.
  • Chabert Pascal
  Journal of Physics D: Applied Physics, IOP Publishing, 2012, 45, pp.495204. A one-dimensional hybrid analyticalnumerical global model of atmospheric pressure radio-frequency (rf) driven capacitive discharges, previously developed, is compared with a basic global model. A helium feed gas with small admixtures of oxygen is studied. For the hybrid model, the electrical characteristics are calculated analytically as a current-driven homogeneous discharge. The electron power balance is solved analytically to determine a time-varying Maxwellian electron temperature, which oscillates on the rf timescale. Averaging over the rf period yields effective rate coefficients for gas phase activated processes. For the basic global model, the electron temperature is constant in time and the sheath physics is neglected. For both models, the particle balance relations for all species are integrated numerically to determine the equilibrium discharge parameters. Variations of discharge parameters with composition and rf power are determined and compared. The rate coefficients for electron-activated processes are strongly temperature dependent, leading to significantly larger neutral and charged particle densities for the hybrid model. For small devices, finite sheath widths limit the operating regimes to low O2 fractions. This is captured by the hybrid model but cannot be predicted from the basic global model. (10.1088/0022-3727/45/49/495204)
  DOI : 10.1088/0022-3727/45/49/495204
• Shell-Reduced MHD: weak and strong turbulence
  
  • Grappin Roland
  • Verdini Andrea
  , 2012, 14, pp.3856. MHD turbulence with guide field B0 is studied using the shell model for Reduced MHD, with the aim of characterizing the scaling, and anisotropy of RMHD at high Reynolds number. A basic prediction of anisotropic turbulence theory (critical balance or CB) is that, in presence of large enough B0, the 1D perpendicular energy spectrum should exhibit a k&#8869;-2 scaling characteristic of weak nonlinear coupling at large scales, followed by a break and a k&#8869;-5/3 scaling at smaller scales. Previous numerical simulations never obtained such double spectra, but instead displayed single power-laws with a monotonous increase of their index when B0 was increased. In the present work we observe for the first time the double scaling, possibly thanks to the high Reynolds number reachable in shell models. A corollary of the CB theory is that a k||-2 spectrum develops in the range with strong k&#8869;-5/3 scaling, characterizing spectral anisotropy. In our study, we find that, although the basic paradigm of the CB are verified, the parallel spectrum differs, showing an over-excitation of parallel modes, due to high-frequencies of the large perpendicular eddies. This proves on the one hand that the CB theory is well adapted to the RMHD model but on the other hand that the model is richer than previously thought.
• Electric and magnetic contributions to spatial diffusion in collisionless plasmas
  
  • Smets Roch
  • Belmont Gérard
  • Aunai Nicolas
  Physics of Plasmas, American Institute of Physics, 2012, 19, pp.102309. We investigate the role played by the different self-consistent fluctuations for particle diffusion in a magnetized plasma. We focus especially on the contribution of the electric fluctuations and how it combines with the (already investigated) magnetic fluctuations and with the velocity fluctuations. For that issue, we compute with a hybrid code the value of the diffusion coefficient perpendicular to the mean magnetic field and its dependence on the particle velocity. This study is restricted to small to intermediate level of electromagnetic fluctuations and focuses on particle velocities on the order of few times the Alfvén speed. We briefly discuss the consequences for cosmic ray modulation and for the penetration of thermal solar wind particles in the Earth magnetosphere. (10.1063/1.4762845)
  DOI : 10.1063/1.4762845